Dosage-dispensing devices and dosage-dispensing methods of the aforementioned kind are commonly known in many fields of industry and have been in use for years. They include in most cases a gravimetric measuring instrument, specifically a weighing system, by means of which the dispensed mass of dosage material is measured. The amounts of mass to be measured range from tons down to the smallest quantities of a few micrograms. Especially the dispensing of minuscule amounts, for example in the development of new active ingredients, requires the highest precision, as even the smallest deviation of the mass of the active ingredient can strongly affect the experiments that are performed after the dosage-dispensing process. For instance, when substances are mixed together, the reaction rates can vary considerably as a result of mass deviations; or in clinical tests, the effects on the organism of the test person can deviate considerably from the expected outcome, to name only a few examples.
In order to perform the experiments, one has to prepare a large number of samples. For example, small amounts of pulverous substances are dispensed into a target container, where they are dissolved by adding a solvent. The sample which has been prepared in this manner is subsequently analyzed, for example in an HPLC (High Performance Liquid Chromatography) analyzer.
A dosage-dispensing device capable of measuring out minute amounts of mass and a method of optimizing a dosage-dispensing process are disclosed in EP 1 947 427 A1. The dosage-dispensing device includes a weighing system with a load receiver, a processor unit, a memory unit, and an exchangeable dosage-dispensing unit. The dosage-dispensing unit is arranged above the load receiver on which a target container can be set in place. So that target containers of different heights can be used, the dosage-dispensing unit is height-adjustable in relation to the load receiver.
Extraneous influences play an important part when extremely small masses are measured out. Air movements, temperature fluctuations and the like can strongly influence the weighing result or, more specifically, the weight values determined by the weighing system. If these weight values are used for the control of the dosage-dispensing process and as a basis for the amount or mass of solvent that is to be subsequently added, the aforementioned extraneous factors can lead to faulty samples.
When measuring out pulverous substances with the dosage-dispensing device of the foregoing description, it was found that electrostatic effects, too, can introduce significant errors into the weighing results. If the target container and/or the dosage-dispensing unit include materials that are not electrically conductive, they can become electrostatically charged. This can have the consequence that the target container and the dosage-dispensing unit mutually repel or attract each other. Accordingly, the weighing system will measure values that are higher or lower than the mass that is actually present in the target container.
As a solution to this problem, ionizers are being offered by means of which the ambient air of the dosage-dispensing unit and the target container is ionized, whereby the static charges can be effectively eliminated. However, as these ionizers use high voltage levels and therefore generate an electromagnetic field, their operation can likewise have a harmful influence on the weight values of the weighing system. Furthermore, the ion flow of the ionizer can set air masses into motion and the latter can exert a force on the load receiver.
In a further approach to solve this problem, all components of the dosage-dispensing device, the dosage-dispensing unit and the target container are made electrically conductive and are connected to ground. This usually leads to very good results. However, it involves the application of metallic coatings or the fabrication of metal parts, which increases the cost considerably, since non-conductive materials have to be coated with a layer of conductive material, or the respective parts have to be made of metal.
In addition, extensive experiments have shown that the ability to accumulate electrostatic charges is not limited to the dosage-dispensing unit and/or the target container. There are also some pulverous substances which are critical in regard to their electrostatic behavior and can build up a significant electrostatic charge as a result of break-up and friction effects during the dispensing process. Even with the use of electrically conductive, grounded target containers or dosage-dispensing units and/or an ionizer, no satisfactory solution has been found for this problem. For example, when measuring out paracetamol (also known as acetaminophen) in dosage quantities of 12 mg, deviations due to electrostatic effects could be observed which amounted to as much as 40% of the specified mass.